Insulated-gate bipolar transistors (IGBT) generally include an N.sup.+ -type buffer region positioned between a P.sup.+ -type region in a substrate and an N.sup.- -type drift region in the substrate. A gate and an emitter are formed in the surface of the substrate in the drift region and a collector is formed in a lower surface of the substrate in the P.sup.+ -type region.
In the prior art, the buffer region is formed in one of two ways: either an N.sup.+ -type buffer layer is epitaxially grown on a P.sup.+ -type substrate and an N.sup.- -type drift region is then epitaxially grown on the N.sup.+ -type buffer layer; or a thin (10 mil) N-type wafer is used, and a P-type dopant is implanted into the back side of the wafer to form a collector.
The problem with the first method is that thin, heavily doped layers are difficult to grow epitaxially. As one example, the high temperatures required in epitaxial growth tend to alter the doping profile of previously grown layers.
The problem with the second method is that thin wafers are difficult to handle. In addition, this method produces IGBTs that are more suitable for very high voltage devices, such as 1200 voltage and 1800 voltage devices.
A major problem in the prior art is that the N.sup.+ -type buffer region has a doping concentration which is naturally limited by the processes to less than 1.times.10.sup.17 /cm.sup.3 and a thickness of 10 to 20 microns. A doping concentration of less than 1.times.10.sup.17 /cm.sup.3 is not high enough to substantially reduce the hole injection from the P.sup.+ -type region into the N.sup.- -type drift region. As a result, a significant amount of holes still is accumulated in the drift region and the device is slow in turning OFF because of this stored charge.
To increase the switching speed of the conventional IGBTs, lifetime control techniques, such as electron irradiation must be used during the device processing. Such additional processing results in higher processing costs and lower yield. Also, electron irradiation has a tendency to damage the micro-structure of the material and reduce the useful life of the devices.
Thus, it would be advantageous to devise a fabrication method in which a buffer region is formed that substantially prevents significant amounts of holes from being accumulated in the drift region.
Accordingly, it is a purpose of the present invention to provide new and improved fast-switching, low-R(ON) insulated-gate bipolar transistors.
It is another purpose of the present invention to provide new and improved methods of fabricating fast-switching, low-R(ON) insulated-gate bipolar transistors.
It is yet another purpose of the present invention to provide new and improved methods of fabricating fast-switching, low-R(ON) insulated-gate bipolar transistors which are more controllable and reproducible.
It is a further purpose of the present invention to provide new and improved methods of fabricating fast-switching, low-R(ON) insulated-gate bipolar transistors which do not require the use of lifetime control techniques, such as electron irradiation, during the device processing.